On Sun, 6 Feb 2022 04:34:19 +0300 Vitaly Chikunov <v...@altlinux.org> wrote:
> `struct dirent' returned from readdir(3) could be shorter (or longer) > than `sizeof(struct dirent)', thus memcpy of sizeof length will overread > into unallocated page causing SIGSEGV. Example stack trace: > > #0 0x00005555559ebeed v9fs_co_readdir_many (/usr/bin/qemu-system-x86_64 + > 0x497eed) > #1 0x00005555559ec2e9 v9fs_readdir (/usr/bin/qemu-system-x86_64 + 0x4982e9) > #2 0x0000555555eb7983 coroutine_trampoline (/usr/bin/qemu-system-x86_64 + > 0x963983) > #3 0x00007ffff73e0be0 n/a (n/a + 0x0) > > While fixing, provide a helper for any future `struct dirent' cloning. > > Resolves: https://gitlab.com/qemu-project/qemu/-/issues/841 > Cc: qemu-sta...@nongnu.org > Co-authored-by: Christian Schoenebeck <qemu_...@crudebyte.com> > Reviewed-by: Dmitry V. Levin <l...@altlinux.org> > Signed-off-by: Vitaly Chikunov <v...@altlinux.org> > --- > Tested on x68-64 Linux with btrfs-progs tests and slow qos-test. > Changes since v3: > - Update commentary on qemu_dirent_dup logic. > - Use g_memdup as suggested by Greg Kurz. > The synth driver should really be made compliant with the _DIRENT_HAVE_D_* macros at some point, but this patch fixes a crash and the comment is clear enough to understand the sz == 0 weirdness so I won't object any further. Acked-by: Greg Kurz <gr...@kaod.org> > hw/9pfs/codir.c | 3 +-- > include/qemu/osdep.h | 13 +++++++++++++ > util/osdep.c | 21 +++++++++++++++++++++ > 3 files changed, 35 insertions(+), 2 deletions(-) > > diff --git a/hw/9pfs/codir.c b/hw/9pfs/codir.c > index 032cce04c4..c0873bde16 100644 > --- a/hw/9pfs/codir.c > +++ b/hw/9pfs/codir.c > @@ -143,8 +143,7 @@ static int do_readdir_many(V9fsPDU *pdu, V9fsFidState > *fidp, > } else { > e = e->next = g_malloc0(sizeof(V9fsDirEnt)); > } > - e->dent = g_malloc0(sizeof(struct dirent)); > - memcpy(e->dent, dent, sizeof(struct dirent)); > + e->dent = qemu_dirent_dup(dent); > > /* perform a full stat() for directory entry if requested by caller > */ > if (dostat) { > diff --git a/include/qemu/osdep.h b/include/qemu/osdep.h > index d1660d67fa..ce12f64853 100644 > --- a/include/qemu/osdep.h > +++ b/include/qemu/osdep.h > @@ -805,6 +805,19 @@ static inline int platform_does_not_support_system(const > char *command) > } > #endif /* !HAVE_SYSTEM_FUNCTION */ > > +/** > + * Duplicate directory entry @dent. > + * > + * It is highly recommended to use this function instead of open coding > + * duplication of @c dirent objects, because the actual @c struct @c dirent > + * size may be bigger or shorter than @c sizeof(struct dirent) and correct > + * handling is platform specific (see gitlab issue #841). > + * > + * @dent - original directory entry to be duplicated > + * @returns duplicated directory entry which should be freed with g_free() > + */ > +struct dirent *qemu_dirent_dup(struct dirent *dent); > + > #ifdef __cplusplus > } > #endif > diff --git a/util/osdep.c b/util/osdep.c > index 42a0a4986a..67fbf22778 100644 > --- a/util/osdep.c > +++ b/util/osdep.c > @@ -33,6 +33,7 @@ > extern int madvise(char *, size_t, int); > #endif > > +#include <dirent.h> > #include "qemu-common.h" > #include "qemu/cutils.h" > #include "qemu/sockets.h" > @@ -615,3 +616,23 @@ writev(int fd, const struct iovec *iov, int iov_cnt) > return readv_writev(fd, iov, iov_cnt, true); > } > #endif > + > +struct dirent * > +qemu_dirent_dup(struct dirent *dent) > +{ > + size_t sz = 0; > +#if defined _DIRENT_HAVE_D_RECLEN > + /* Avoid use of strlen() if platform supports d_reclen. */ > + sz = dent->d_reclen; > +#endif > + /* > + * Test sz for zero even if d_reclen is available > + * because some drivers may set d_reclen to zero. > + */ > + if (sz == 0) { > + /* Fallback to the most portable way. */ > + sz = offsetof(struct dirent, d_name) + > + strlen(dent->d_name) + 1; > + } > + return g_memdup(dent, sz); > +}
